Complex perovskite compounds have dielectric properties and/or piezoelectric properties, therefore have been widely utilized for dielectric materials, piezoelectric materials etc., and also have been utilized as materials of thermoelectric conversion elemental devices (hereinafter referred to as “thermoelectric conversion material”). The thermoelectric conversion means the conversion between thermal energy and electric energy through Seebeck effect or Peltier effect. By using of the thermoelectric conversion, it is possible to generate electric power from heat flow using the Seebeck effect or to cause endothermic cooling phenomena by flowing an electric current using Peltier effect. Single element units formed of metals or semiconductors are typically utilized for the thermoelectric conversion elemental devices, their performance indices depend on higher order structures such as crystallinity. Therefore, compounds with less structural defects are required for the thermoelectric conversion materials in order to provide the single element units with higher performance indices.
Examples of the preferable compounds for such thermoelectric conversion materials are Bi—Te, Pb—Te and Si—Ge compounds etc. Among these, Bi—Te and Pb—Te compounds may exhibit excellent thermoelectric properties at around room temperature and moderate temperatures of 300 to 500° C. However, these compounds have poor thermal resistance at higher temperatures, thus are difficult to be used at higher temperatures. Furthermore, there are such problems that these compounds contain expensive rare elements such as Te, Sb and Se, therefore their production costs are likely to be higher and also contain environmental-load elements such as Te, Sb, Se and Pb having an intensive toxicity.
Contrary to this, the thermoelectric conversion materials of oxide ceramics contain no rare elements or environmental-load elements, and have features that the thermal resistance is higher due to less structural defects and the degradation of thermoelectric properties is lower at higher temperatures under prolonged use thereof; accordingly, they are attracting attention as alternate materials of compound semiconductors. Perovskite-type compounds of CaMnO3, for example, are proposed in which 10% of Ca sites are replaced by metal elements such as Bi, La and Ce (see Non-Patent document 1). In addition, inexpensive, thermally stable and less environmental-load cobalt-containing oxides are also attracting attention.
Non-Patent Document 1: Michitaka Ohtaki et. al., Journal of Solid State Chemistry 120, 105-111 (1995)